This invention relates to a method and apparatus for soldering aluminum, and more particularly to a method and apparatus for making solder repairs in aluminum articles, such as automobile radiators, heat exchangers, and the like, with the use of ordinary soft solder and conventional soldering techniques.
There are many services where aluminum can advantageously be employed because of its availability, metallurgical and physical characteristics, good heat exchange capacity, and light weight. A large market for aluminum exists in the field of radiators and other heat exchangers used in automotive vehicles. Acceptance of aluminum for purposes such as automobile radiators, however, depends upon the availability of simple and inexpensive techniques for repairing leaks and other damage in the radiators or the like, and it is essential that the techniques used in repair do not require the use of unusual or expensive equipment and do not require highly skilled operators or unusual working conditions.
At the present time in the United States, there are several thousand automobile radiator repair shops. The operators of these shops are accustomed to repairing copper-brass radiators by the use of conventional soft soldering techniques, but these techniques heretofore have not been successful in connection with the soldering of aluminum.
The problem is that when soft solder is used in connection with aluminum, interfacial corrosion occurs very rapidly where electrolytes are present. With usual soft soldering methods, the bond between the solder and the underlying aluminum surface has not been good enough to prevent entirely the penetration of electrolyte to the interface and corrosion occurs. Various methods have been proposed to overcome this difficulty. For example, an abrasive soldering technique has been proposed in an effort to remove all of the oxide from the aluminum surface. This requires zinc solder and hard brushing of the joint while the molten zinc solder is on the leak area. This method also requires that care be taken to avoid overheating of the parts.
Electroplating techniques have been proposed to enable the use of low melting point solders. Techniques employing two or more electrodeposited layers have been used. One such method consists in first electroplating a layer of copper and then plating a layer of nickel over the copper. These layers have thicknesses in the order of tenths of thousandths of an inch. The time required to deposit the two layers is substantial and, although the soldering step can be carried out by conventional methods, stresses built up in the two layers themselves in service may ultimately result in loss of adhesion and ultimately interfacial corrosion taking place.
It has also been proposed to utilize a coating of nickel on which a thin coating of gold is electrodeposited. The use of gold plating greatly increases the cost of the operation and in this system also, there may be separation of the electroplated coatings in service. Various soldering methods are described in an article entitled "The Repair of Aluminum Heat Exchangers" by R. A. Morley and T. L. Wilkinson, appearing in the October 1978 issue of Welding Journal. Aluminum soldering techniques in general are also described in Aluminum Soldering Handbook, Third edition, 1976, published by the Aluminum Association, Inc., New York, N.Y.